The invention concerns an apparatus for the drying of particulate materials in superheated steam in a closed container which is configured as a revolution element. The container has a lower cylindrical part which via a conical transition piece is connected to an upper cylindrical part with a greater diameter. In a centre part of the container there is a heat exchanger, and below this an element for the transport of steam, e.g. in the form of a blower such as a centrifugal blower. The container comprises a series of upwardly-open, elongated and substantially vertical processing cells which are disposed around the central part with the heat exchanger. The last of these processing cells has a closed bottom and is the discharge cell, while the remainder have a bottom through which steam can permeate. The processing cells, which lie at the side of one another, are open at the top opposite a common transfer zone, and at the bottom stand in mutual connection through openings at the lower ends of the cells. The particle-formed material is led into the first of the processing cells, and is dried during its passage through the processing cells by the superheated steam which by the steam transport element is blown up from the heat exchanger through the permeable bottom of the cells, in that the particle-formed material can pass from one processing cell to the next through said openings. The upper cylindrical part also contains a dust separation system in the form of a cyclone for the cleaning of the steam before this is transported further.
The material to be dried is led into the first of the processing cells where it is brought into a swirling movement by the steam which flows up through the steam-permeable bottom of the cell. The heaviest particles pass from the one processing cell to the next through openings at the bottom. The lighter particles are blown up into the conical part, which is similarly divided into cells. Moreover, these cells are divided by inclined plates which form conical surfaces. Opposite the lowermost parts of the conical surfaces there are openings between the processing cells to which material is led by guide rails placed on the conical surfaces. Above the cells there is a common zone where material is also fed forwards towards the discharge cell. Unlike the remaining cells, steam does not flow up through the bottom of the discharge cell. Consequently, all of the product which reaches this cell falls down to the bottom, from where it is led away.
An apparatus of this type is known, for example from DK patent publication no. 156 974, EP patent publication no. 537 262 and EP patent publication no. 537 263.
The use of the apparatus for the drying of sugar beet pulp is discussed in an article by Arne Sloth Jensen in International Sugar Journal, November 1992, Vol. 94, No. 1127. The dried beet pulp is normally used as cattle feed. It is precisely within the sugar industry that the apparatus finds particular application. In this as well as in other industries, the apparatus enables the drying to take place without oxidising the product and without any influence on the environment, in that the drying is effected in a closed container, in this case under pressure. Consequently, nothing escapes to the atmosphere, unlike the conventional drum-type dryers, which can be smelled approx. 20 km away. The water which is removed from the moist product leaves the drier as steam. This steam contains all of the energy which is used for the drying, and it can be used in the factory as processing steam. A normal sugar factory hereby saves between 50 and 120 tons of fuel oil per day, or a corresponding amount of other fuel. Moreover, the process makes it possible for a sugar factory to keep the whole of the production running with bio-fuel by burning the dried waste from the process, this waste in dried form containing more energy than the sugar factory requires. In such a case, the saving in the amount of fuel is approx. three times greater.
The known apparatus can also be used for the drying of wood-chips or other moist fuels, whereby the overall energy savings are increased.
It is desirable, however, for the capacity of the apparatus to be increased, so that the capacity is increased in proportion to the cost of the apparatus, in that the relatively high price of the known apparatus in relation to its capacity is the most significant disadvantage of the known apparatus.
With the known apparatus, the capacity is more or less proportional to the circulating flow of steam. With the hitherto known configuration of the apparatus, where the supply of steam to the cyclone takes place at the bottom of the cyclone, the flow can not be increased without this at the same time resulting in an unacceptable great amount of particle-formed material being swept up with the steam into the dust-separation cyclone. From here, it will pass out of the apparatus without having been adequately dried, and thus the quality of the product discharged is reduced.
It is thus the object of the invention to provide an apparatus which has a greater drying capacity than the known types of apparatus, without this giving rise to an increase in the cost of the apparatus, and without any reduction in the quality of the finished product.
This object is achieved in that at least a larger part of the steam supply from the common transfer zone to the cyclone is effected in an upper part of the cyclone.
The apparatus can hereby operate with a greater circulating flow of steam, in that the large volume in the container around the dust-separation cyclone is involved in the separation. This is effected by not feeding the steam, or only to a small extent, into the bottom of the cyclone, which has hitherto been the practice, but by feeding at least a larger part of the steam, i.e. at least a half part, into the upper part of the cyclone. It has thus proved, surprisingly, that the supply of steam to the bottom of the cyclone can be closed without this giving rise to a blockage. With the apparatus according to the invention, the moist product material which is carried out of the top of the processing cells, and especially the first of the processing cells, will not reach the cyclone. Instead, and as a result of the centrifugal force which arises when the particles are carried around with the flow of steam in the uppermost part of the container around the cyclone and forward towards the cyclone""s steam supply, these particles will hit the outer wall of the container. Here they will form a layer which will slide downwards and back to the processing cells. It will thus only be dried dust which is carried with the steam flow into the cyclone.
It has thus shown with the invention that the steam flow can be increased to such a degree that the capacity of the apparatus is increased by 20-25% without an increase in the cost of the apparatus, and without any reduction in the quality of the finished product.
With a suitable configuration, the supply of steam from the common transfer zone to the cyclone can take place in an area which lies substantially directly above the last cells, i.e. the last processing cells and the discharge cell. It is hereby further ensured that moist particles carried from the processing cells and especially from the first processing cells will not be able to pass directly into the cyclone, but will be driven around this so that a separation of these particles takes place.
A smaller part of the steam flow, i.e. less than a half part, can be supplied to the lower part of the cyclone, but it can also be chosen to let the whole of the steam supply take place in the upper part of the cyclone.
The separation of particles, which is effected in the volume around the cyclone, can be reinforced by suspending cylindrical or spiral plates down from the top of the container, so that the plates are disposed wholly or partly around between these concentric or spiral plates forwards towards the cyclone, a layer of particles will be formed on the inner sides of the plates, and this will slide down and back to the precessing cells.
It can be expedient for openings to be formed in the cylindrical plates, so that the steam can flow forward to the steam supply opening in the cyclone.
It can be expedient for the bottom of the cyclone to be configured with a discharge opening for the separated dust, and this discharge opening can also be connected with a pipe, said pipe leading the separated dust down into the discharge cell, from where the dust is led out together with the remaining dried product material.